With the development of technologies and the continuous increase of people's needs, the acquisition of information from the outside world becomes increasingly important. From the earliest black and white photos to the color photos, to the videos that records temporal information, the means to record and show the world continuously improve. 3D technologies that have appeared in recent years greatly improve the ways that humans perceive the world. 3D movies, naked eye TV, virtual reality, augmented reality, and other applications greatly enrich people's lives, and have made some scientific researches more convenient. The critical difference from previous applications is that these applications have additional depth information, which can create 3D visual experiences and enhance the sense of presence. Therefore, depth information has become hot topic for research.
There are a variety of methods to obtain the depth information, which are mainly divided into two types: contact 3D scanning and non-contact 3D scanning. A contact 3D scanner measures 3D coordinates and other information mainly by actual contacts with the measured object, which obtains its depth information. Although this method features high accuracy, contacting a measured object may cause damage to the object. The method is also time-consuming. It is thus seldom used. The other method is non-contact 3D scanning, which can obtain the depth information without contact with the measured object. This method can include active scanning and passive scanning. In active scanning, the depth information is measured by actively transmitting signals or energy to the measured object. In passive scanning, the depth information is obtained via the image information without transmitting energy. Common active scanning methods include time difference ranging, triangulation, and others using a laser range finder, as well as structured light source method via image projection. Common passive scanning methods include stereo matching, the chroma method, and others, which are implemented using algorithms.
Both active scanning and passive scanning generates a depth map corresponding to the measured scene, which is a grayscale image that indicates the depths of objects by color density. From the above descriptions, it is easy to understand that the qualities of the depth maps have a huge impact on later applications. However, the depth maps obtained via the existing methods have various problems such as black holes, irregular edge of objects, etc. For the depth map obtained by active scanning, the noise is generally removed from the depth image by filtering. Relative to active scanning, the stereo matching in passive scanning includes an additional view angle. Thus these depth maps can be repaired using information of the two view angles. In general, the left-right consistency checks are used to detect inconsistent regions, such regions being subsequently treated by filtering, etc. Although depth maps (or a disparity map) of stereo matching are more detailed after processing than active scanning, some black holes and irregular edges still exist.
As mentioned above, the depth information has become a critical technology for many current frontier fields and new applications, which attracts wide attention. Although methods are available for obtaining depth information, due to technical constraints, depth map still includes many quality problems. There have been some methods relating to post-processing of depth maps, but black holes, irregular edges and other artifacts still exist on the depth images after processing, which seriously affect subsequent applications. There is therefore still an urgent need for improving post-processing of depth maps.